JP 2009-144751 A. published by the Japan Patent Office in 2009, proposes that, in a continuously variable transmission (CVT) for a vehicle, in which an endless torque transmission member such as a V-chain is looped around a pair of pulleys including a primary pulley and a secondary pulley, a speed change response is enhanced by appropriately setting a rigidity of the pulleys.
Each of the primary pulley and the secondary pulley changes a width of a V-shaped groove in accordance with a pulley thrust applied in a direction of a rotation axis to change a winding radius of the V-chain. In this manner, a ratio between rotation speeds of the pulleys, that is, a speed ratio is changed. The pulley thrusts are obtained by a hydraulic pressure of a hydraulic pump using an internal combustion engine mounted in a vehicle as a power source.
In the CVT described above, if a slip occurs between the V-chain and any one of the pulleys, torque transmission between the pulleys is adversely affected. In order to prevent the slip from occurring between the V-chain and the pulleys, a predetermined thrust is required to be applied to the pulleys. The predetermined thrust is hereinafter referred to as a slip limit thrust. The slip limit thrust is a value common to the primary pulley and the secondary pulley, and is determined depending on the winding radius of the V-chain around the primary pulley. The slip limit thrust is calculated by the following Expression (1).
                              F          ⁢                                          ⁢          min                =                                            Tp              ·              cos                        ⁢                                                  ⁢            α                                2            ·            μ            ·            Rp                                              (        1        )            
where, Fmin=slip limit thrust,                Tp=input torque to primary pulley,        α=sheave angle,        μ=friction coefficient of V-chain and pulleys, and        Rp=winding radius of V-chain around primary pulley.        
In this expression, the sheave angle α is a constant that is determined in advance by shapes and sizes of the primary pulley, the secondary pulley, and the V-chain. The friction coefficient μ is a constant, which is determined in advance by materials of the primary pulley, the secondary pulley, and the V-chain.
Referring to FIG. 11, in the CVT that transmits torque through the V-chain, when the V-chain stretches, the winding radii around the pulleys change at the same speed ratio. The following Expression (2) expresses the relationship between the speed ratio and the winding radii.
                    ip        =                                            Rs              ⁢                                                          ⁢              1                                      Rp              ⁢                                                          ⁢              1                                =                                    Rs              ⁢                                                          ⁢              2                                      Rp              ⁢                                                          ⁢              2                                                          (        2        )            
where, ip=speed ratio,                Rp1=winding radius around primary pulley when stretch of V-chain is small,        Rs1=winding radius around secondary pulley when stretch of V-chain is small,        Rp2=winding radius around primary pulley when stretch of V-chain is large, and        Rs2=winding radius around secondary pulley when stretch of V-chain is large.        
As can be understood from Expression (1), when the V-chain stretches, the winding radius of the V-chain around each of the pulleys increases. As a result, the slip limit thrust becomes smaller.
Therefore, if the pulley thrusts are controlled based on the slip limit thrust that is calculated without taking the stretch of the V-chain into consideration, the pulley thrusts become excessively large in the case where the V-chain stretches. As a result, a loss in hydraulic pressure or a friction loss increases, which leads to an increase in fuel consumption of the internal combustion engine, which drives the hydraulic pump.
It is therefore an object of this invention to realize appropriate pulley thrust control in accordance with whether or not an endless torque transmission member stretches.